Precise ro-vibrational analysis of molecular bands forbidden in absorption: The ν8+ν10 band of 13C2H4

- High resolution analysis of the 13C2H4 spectra.
- Resonance interactions in asymmetric top molecules.
- Determination of spectroscopic parameters.

The high resolution spectra of the 13C2H4 molecule was recorded with a Bruker IFS 120 Fourier transform spectrometer and theoretically analyzed in the 1650−1800cm−1 region of the ν8+ν10 band which is forbidden in absorption. About 1200 experimental transitions with the maximum values of quantum numbers Jmax.=34 and Kamax.=17 were assigned to the ν8+ν10 band. On that basis the 516 high accuracy ro-vibrational energies of the (v8=v10=1) vibrational state, as well as energy levels with J≤2 of the (v4=v8=1) and (v7=v8=1) vibrational states, were determined which then were used as input data in the weighted fit of spectroscopic parameters of the Hamiltonian (strong local resonance interactions of the ν8+ν10 band with the bands ν4+ν8 and ν7+ν8 have been taken into account). A set of 34 vibrational, rotational, centrifugal distortion, and resonance interaction parameters was obtained from the fit. These parameters reproduce positions of about 1200 experimentally recorded and assigned transitions with the rms error drms=0.00018cm−1 (blended and very weak transitions are not taken into account in that case).